Large wood stoves which are generally called "large box" wood stoves have acquired a reputation of not performing well at either high or low burn rates. One of the reasons for this is that a large stove is required to perform in a much bigger window of burn rate than the common small or mid-size units that operate in a smaller range. Existing wood stoves provide a combustion air system which is generally two or three separate air supplies, some of which may have set openings. Thus, the chosen ratio for set openings may be ideal for a particular burn, for example, a high burn, but it provides an air mixture that is only barely adequate for a low burn. Thus, the narrow window of optimum performance for a large wood stove often yields a stove that has difficulty passing emission standards and in some cases is unresponsive or uncontrollable for in-house use.
It is considered that people buy large wood stoves for two main reasons. One is to have high output at maximum burns and the other is fire holding properties at low burns with good efficiencies at both high and low burns.
With combustion air supply systems having set openings, it has been found that a stove with a minimum set opening large enough to pass the United States EPA regulations for low emissions at minimum burn, provides too much air for normal use. Conversely, if the stove is able to produce a satisfactory extended low burn in the home, then the EPA test results would be poor if met at all.
To overcome this problem, some type of internal self-regulation to vary one or more of the air supplies at different burn rates has been considered. It is generally felt that incorporating electronics, i.e., thermocouples, solenoids, power supplies, logic boards, etc., would be achievable but at high cost and consumers generally would not accept electronics on a wood stove. Thus, simple bi-metal elements have been used on wood stoves for many years. These are cheap to buy, do not require any external power supply and are accepted by the consumer. Examples of wood stoves with bi-metallic thermostats or strips to pivot inlet dampers are disclosed in U.S. Pat. No. 4,136,662 to Willson, U.S. Pat. No. 4,214,569 to Heine, U.S. Pat. No. 4,117,824 to McIntire et al, U.S. Pat. No. 4,409,956 to Barnett, U.S. Pat. No. 4,677,965 to Duerichen and U.S. Pat. No. 4,265,213 to Gorsuch et al.
As will be seen for most of these wood stoves, bi-metal systems control the overall air supply, not a selected portion. Furthermore, traditionally all bi-metals are configured to close air inlets when the temperature rises. Thus, as the temperature rises, the fuel consumption increases, the bi-metal senses the temperature rise and begins to close the air inlet. With less air entering, the overall stove temperature starts to decrease, the bi-metal senses the temperature drop and gradually opens the air inlet. Most bi-metals are connected directly to a flap or gate which continuously fluctuates or oscillates, thus the stove does not remain at a constant temperature.
It is an aim of the present invention to provide a wood stove that has a wide range of burn rates that comply with the United States Environmental Protection Agency emission regulations and also offer "user friendly" characteristics to a consumer. It is a further aim of the present invention to provide a wood stove that does not have the traditional fluctuating or oscillating control function which causes the stove temperature to continuously rise and fall.